My mom was kidnapped by my older sibling whom she hasnt seen in 20 years. My mom who was undergoing memory testing for Dementia told one of her sisters she was being abused. That sister on other hand whom I thought was helping organized the whole kidnapping. Of course my mom is vulnerable and she went freely. I found my mom six month later in a new state starting a whole new life. And all the family member called me ugly names made threats to kill me becasue they believed mom when I was trying to share she has memory loss. Matters worst my sister makes her file for divorce from her husband of 52 years of marrige. My mom is presenting very well in her new state they don't believe she has any memory loss. I was in the process of guardianship.
You really can do nothing at this point. Its her word against yours. And if you have POA, unless its "immediate" you can do nothing unless she is declared incompetent. If they spend her money, then they will pay the piper when she is turned down for Medicaid because they took her money. But please, don't allow yourself to be drawn in. Think really hard about caring for her on ur own or financially. Your future is at stake too. Hope her husband (guessing he is not ur father) got a good lawyer.
All u can do for now is send her cards maybe flowers telling her u love her and think about her often. And sit back and allow the drama to unfold. They will be contacting you saying its ur time to care for Mom. This will be after they go thru her money and find they can't care for someone suffering from Dementia. Thats when u either say "u made ur bed" or u take on the responsibility.
At this point there isn't much you can do, except back off and gain her trust again, and check on her later to make sure she is OK. They probably think that they rescued her. If there is any way you can be with her or talk to her alone at some point in the future, then maybe you can find out where she is with all this. Don't accuse or judge yet, that will just make her defensive. If she is losing her memory to the point that she needs a guardian, then it will become apparent soon enough to her saviors. If at all possible, find a way to keep an eye on her finances, but don't do or touch anything without permission. If you see large transfers of money or anything alarming, then contact the authorities.
Meanwhile get some legal advice, perhaps with your dad along, if he is capable.